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In fact, the logic is so incredibly simple, it's scary: If one row is modified, the statistic is updated. [IX_Person_Last Name_First Name_Middle Name] has been updated… 1 index(es)/statistic(s) have been updated, 2 did not require update. Updating [Sales].[Sales Reason] [PK_Sales Reason_Sales Reason ID] has been updated… [_WA_Sys_00000013_75A278F5], update is not necessary… [_WA_Sys_00000014_75A278F5], update is not necessary… [_WA_Sys_0000000D_75A278F5], update is not necessary… [_WA_Sys_0000000C_75A278F5], update is not necessary… 1 index(es)/statistic(s) have been updated, 7 did not require update. And again, the drawback of that is that maybe resources were used that didn’t need to be.1 index(es)/statistic(s) have been updated, 0 did not require update. Updating [Production].[Product] [PK_Product_Product ID], update is not necessary… If a statistic only has one row modified, should it be updated? If it has 10,000 rows updated, should it be updated? If the table only has 5,000 rows, then absolutely; if the table has 1 million rows, then no, as only one percent of the table has been modified.You can use the aforementioned query that interrogates Realize that different tables may have different thresholds and you will need to tweak the query above for your databases.For some tables, waiting until 15% or 20% of the rows have been modified may be ok.But for others, you may need to update at 10% or even 5%, depending on the actual values and their skew. As much as we love absolutes, they rarely exist in SQL Server and statistics is no exception.You still want to leave Auto Update Statistics enabled – it’s a safety that will kick in if you miss something, just like Auto Growth for your database files.
Statistics are SQL Server objects which contain metrics on the data count and distribution within a column or columns used by the optimizer to help it make that choice. Index statistics: Created automatically when an index (both clustered and non-clustered) is created.
In addition, remember that updating statistics invalidates query plans that use those statistics.
When those queries execute, plans be re-generated, and the plan will probably be exactly the same as it was before, because there was no significant change in the histogram.
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When I was in Chicago a few weeks ago for one of our Immersion Events, an attendee had a statistics question.